Although channel estimation has been widely studied in the conventional wireless communication system, there are two main obstacles for conventional schemes to be directly applied in the RIS assisted system [1]}. Firstly, all RIS elements are passive, which cannot transmit, receive, or process any pilot signals to realize channel estimation. Secondly, since an RIS usually consists of hundreds of elements, the dimension of channels to be estimated is much larger than that in conventional systems, which will result in a sharp increase of the pilot overhead for channel estimation. Therefore, channel estimation is a key challenge in the RIS assisted system, which will be investigated in this invited paper composed of two parts.
